This research project consist of acoustic and biomechanic investigations of the function of the human larynx in phonation. Specific aims are (1) to develop a quantitative theory of fundamental frequency (Fo) control in phonation, (2) to determine the sources of irregularity of vocal fold vibration, (3) to discover the mechanisms of voluntary and involuntary register transitions, (4) to study the relationship between vocal fold hydration and phonation threshold pressure, (5) to study the relationship between mechanical stress in vocal fold tissue and modes of vibration, and (6) to design and construct an artificial larynx for research purposes. Together, the studies constitute an investigation of the physical and biomechanic properties of the major sound source in speech production. The outcome of the studies will have direct impact on diagnosis and treatment of voice disorders. Experimental procedures include the use of laboratory animals (compliance with recent animal care recommendation) and human subjects whenever necessary, but the bulk of the experimentation is done on physical and computer simulation models. In particular, a finite-element simulation model of vocal fold vibration is used to integrate fragmentary pieces of information obtained in isolated experiments. This allows for a high degree of theoretical interpretability of the data obtained from animals and human subjects.